High-frequency content boost for vibratory seismic source and method

ABSTRACT

Computer software, controller and method for generating a desired pilot signal for driving a vibratory source. The method includes steps for selecting a pilot target amplitude spectrum for the vibratory source; determining an initial pilot signal that matches the pilot target amplitude spectrum; associating the initial pilot signal with first and second frequency bands, the second frequency band including a high-frequency end of a range of the vibratory source; band-passing a first part of the initial pilot signal associated with the first frequency band with a first band-pass configuration; band-passing a second part of the initial pilot signal associated with the second frequency band with a second band-pass configuration; level compressing the first and second parts of the initial pilot signal; recombining the first and second parts of the initial pilot signal to form a recombined pilot signal; and processing the recombined pilot signal to obtain the desired pilot signal.

BACKGROUND

1. Technical Field

Embodiments of the subject matter disclosed herein generally relate tomethods and systems and, more particularly, to mechanisms and techniquesfor boosting low- and/or high-frequency content for seismic sources.

2. Discussion of the Background

Reflection seismology is a method of geophysical exploration to imagethe subsurface of the earth for determining its properties, whichinformation is especially helpful in the oil and gas industry. Typicallya controlled source sends seismic energy waves into the earth. Bymeasuring the time it takes for the reflections to come back to pluralreceivers, it is possible to estimate the depth and/or composition ofthe features causing such reflections. These features may be associatedwith subterranean hydrocarbon deposits.

For land applications, vibratory sources are commonly used. Vibratorysources, including hydraulically powered sources, electro-dynamic andsources employing piezoelectric or magnetostrictive material, cangenerate signals that include various frequency bands, commonly referredto as “frequency sweeps.” In other words, the frequency band of suchsources may be controlled.

Seismic vibrators in use today have constraints that imposefrequency-variant limits on their output amplitude spectrum. Certainconstraints have been recognized in the art. For instance, Bagaini etal. (U.S. Pat. No. 7,327,633, the entire disclosure of which isincorporated herein by reference), have recognized that massdisplacement (or “stroke”) of a seismic vibrator device imposes aconstraint on the frequency content emitted by the vibratory source.However, while a given constraint, such as a mass displacement, of aseismic vibrator has been considered when designing a sweep forachieving a desired target output spectrum by the seismic vibrator, suchconsideration of a single constraint fails to take into account otherconstraints that may impose limitations on the sweep, and thus theresulting designed sweep may fail to operate properly when implementedon the seismic vibrator.

Thus, Sallas (patent application Ser. No. 12/576,804, herein '804, theentire content of which is incorporated herein by reference), exploresand takes into account various constraints (not only the strokelimitation) that impose frequency-variant limits on the source outputamplitude spectrum. These constraints include but are not limited to:reaction mass stroke, maximum deliverable pump flow, holddown weight,servo-valve response, available supply pressure, and driven structureresponse. The problem is compounded by other effects like absorption ofhigh frequency energy and environmental noise. While a conventionallinear sweep may work well enough to image the subsurface given enoughsweep time, it may not provide the most economical solution especiallyif it requires the use of very long sweep times or many shots at aparticular location.

Thus, '804 disclosed a sweep generator that employs a procedure thatcreates a nonlinear swept sine wave sweep to build up the sweep spectraldensity to achieve a target spectrum (that is defined by the user tomeet the geophysical survey objectives) in compliance with (i.e.,without violating) various constraints of the seismic vibrator. '804also considers other constraints such as environmental constraints(which may be defined by an operator or derived from prior data about atarget location), and the disclosed sweep generator employs a procedurefor determining a sweep (e.g., a nonlinear sweep) to achieve the targetspectrum in compliance with those other constraints in addition to orinstead of the constraint(s) of the seismic vibrator that are accountedfor by the sweep generator.

For example, '804 discloses that when working near populated areas itmay be desirable to reduce the instantaneous peak amplitude of thevibrator force through a certain range of frequencies so as not toexcite some structural resonance. Likewise, the sweep generationtechniques described in '804 may be implemented to compensate for a dropin instantaneous amplitude through a range of frequencies imposed byenvironmental constraints and a suitable nonlinear sweep may begenerated to build up the sweep spectral density to achieve a targetspectrum.

However, the aforementioned techniques for generating sweeps thatcompensate for system constraints that fall in the low- andhigh-frequency range are designed for use with swept sine waveexcitation signals and are not well suited for use with pseudo-randomexcitation signals. Existing techniques that are designed for use withswept sine waves include compensation methods for the low-frequency endthat avoid the possibility of driving the source to reach the strokelimitations, i.e., the reaction mass of the source may reach the stops.At the high-frequency end, especially if it is desired to mimic thespectrum of a non-linear sweep designed to overcome the high-frequencyattenuation of the earth (absorption), the existing techniques reducethe risk of overdriving the servo-valve or even run into overpressuresituations in the actuator that can lead to working fluid cavitation.

With interest in using unconventional sweeps to increase productivitythrough use of separable simultaneous sources, there is a need for acorresponding pilot sweep design method that is suitable for use withpseudo-random pilot sweeps that maximizes the energy in the sweep whilestill honoring the target spectrum without exceeding system constraints:that is, a pilot signal configured to drive a seismic vibratory sourceto avoid the stroke limitations at the low-frequency end and to notoverdrive the servo-valve of the source at the high-frequency end and,at the same time, to boost the low- and high-frequency ends (content) asthese parts of the spectrum are important for imaging the subsurface.

With swept sine wave sweeps the frequencies usually change monotonicallyand at any point in time there is only one frequency or a verynarrowband range of frequencies in the pilot signal. Pseudo-random pilotsweeps impose special problems because at any point in time a pluralityof frequencies is present simultaneously and their subsequent impact onsystem demand more difficult to predict. Furthermore, it is desirablefor the output of the vibrator to follow the pilot signal, sincetypically the pilot signal is used as a correlation operator and assumedto be representative of the emitted energy. The drive level for thesource radiated output is chosen so that the vibrator does not exceedsome system limit within its sweep. Drive level is usually defined as apercentile of the holddown weight or the peak force output rating of thevibrator, where for example a drive level setting of 80% would implythat the peak force output of the truck is 80% of the static holddownforce applied to the baseplate to keep it in contact with the earth. Areduction in the drive level setting will reduce the force output of thetruck throughout the entire sweep time. Reduction in output isundesirable because it will reduce the signal to ambient noise ratio inthe received signal.

If for some reason a pseudo-random pilot signal creates peak demandsthat exceed system limits at any time within the sweep interval, thenthe vibrator drive level setting will need to be reduced resulting in areduction in the total emitted energy. So it is desired that thepseudo-random signal be designed to maximize its energy content withoutcreating peak demands that lead to exceed system constraints so thatdrive level settings can be maximized. Accordingly, it would bedesirable to provide systems and methods that overcome theafore-described problems and drawbacks.

SUMMARY

According to an exemplary embodiment, there is a method for generatingwith a computing device a desired pilot signal for driving a vibratorysource to generate seismic waves. The method includes selecting a pilottarget amplitude spectrum for the vibratory source; determining aninitial pilot signal that matches the pilot target amplitude spectrum;associating the initial pilot signal with first and second frequencybands, the second frequency band including a high-frequency end of arange of the vibratory source; selecting a first band-pass configurationfor the first frequency band and a second band-pass configuration forthe second frequency band; band-passing a first part of the initialpilot signal associated with the first frequency band with the firstband-pass configuration; band-passing a second part of the initial pilotsignal associated with the second frequency band with the secondband-pass configuration; level compressing the first and second parts ofthe initial pilot signal; recombining the first and second parts of theinitial pilot signal to form a recombined pilot signal; and processingthe recombined pilot signal to obtain the desired pilot signal. Thedesired pilot signal boosts the high-frequency end of the range of thevibratory source comparative to the initial pilot signal.

According to another exemplary embodiment, there is a controllerconfigured to generate a desired pilot signal for driving a vibratorysource to generate seismic waves. The controller includes an interfaceconfigured to receive a pilot target amplitude spectrum for thevibratory source; and a processor connected to the interface. Theprocessor is configured to determine an initial pilot signal thatmatches the pilot target amplitude spectrum, associate the initial pilotsignal with at first and second frequency bands, the second frequencyband including a high-frequency end of a range of the vibratory source,select a first band-pass configuration for the first frequency band anda second band-pass configuration for the second frequency band,band-pass a first part of the initial pilot signal associated with thefirst frequency band with the first band-pass configuration, band-pass asecond part of the initial pilot signal associated with the secondfrequency band with the second band-pass configuration, level compressthe first and second parts of the initial pilot signal, recombine thefirst and second parts of the initial pilot signal to form a recombinedpilot signal, and process the recombined pilot signal to obtain thedesired pilot signal. The desired pilot signal boosts the high-frequencyend of the range of the vibratory source comparative to the initialpilot signal.

According to still another exemplary embodiment, there is a computerreadable medium including computer executable instructions, wherein theinstructions, when executed by a processor, implement the method notedabove.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate one or more embodiments and,together with the description, explain these embodiments. In thedrawings:

FIG. 1 is a schematic diagram of a vibratory source according to anexemplary embodiment;

FIG. 2 is a graph illustrating a pilot signal scaled to force andplotted versus time according to an exemplary embodiment;

FIG. 3 is a graph illustrating a first temporal integration of the pilotsignal of FIG. 2 according to an exemplary embodiment;

FIG. 4 is a graph illustrating a double time integration of the pilotsignal of FIG. 1 according to an exemplary embodiment;

FIG. 5 is a graph illustrating a predicted displacement versus ameasured displacement of a reaction mass of a vibratory source accordingto an exemplary embodiment;

FIG. 6 A-B is a graph illustrating the output of a compression functionversus a displacement of a reaction mass of a vibratory source accordingto an exemplary embodiment;

FIG. 7 is a graph illustrating the output of a compression function anda displacement of a reaction mass of a vibratory source versus timeaccording to an exemplary embodiment;

FIG. 8 if a flowchart illustrating a method for boosting thelow-frequency content for a vibratory source according to an exemplaryembodiment;

FIG. 9 is a graph illustrating a target spectrum according to anexemplary embodiment;

FIG. 10 illustrates a pilot signal in a force domain according to anexemplary embodiment;

FIG. 11 illustrates an effect of a displacement domain compression on apilot signal in the force domain;

FIG. 12 is a graph illustrating an original pilot signal and a modifiedpilot signal for boosting the low-frequency content of a vibratorysource according to an exemplary embodiment;

FIG. 13 is a graph showing the high frequency force output limit of asmall hydraulic vibrator;

FIG. 14 is a flowchart of a method for boosting the high-frequencycontent of a vibratory source according to an exemplary embodiment;

FIGS. 15A-F show various spectrum shapes for a vibratory source and apilot signal for driving the same according to an exemplary embodiment;

FIG. 16 is a schematic diagram of a computer system suitable fordesigning pilot sweeps;

FIG. 17 is a flowchart of a method for increasing a low-frequencycontent of a vibratory source according to an exemplary embodiment;

FIG. 18 is a flowchart of a method for increasing a high-frequencycontent of a vibratory source according to an exemplary embodiment;

FIGS. 19A-B illustrated bandpass filtered comparison of pilot signalbefore and after high frequency level compression process; and,

FIG. 20 illustrates pilot amplitude spectrum with high frequency boostafter high frequency level compression.

DETAILED DESCRIPTION

The following description of the exemplary embodiments refers to theaccompanying drawings. The same reference numbers in different drawingsidentify the same or similar elements. The following detaileddescription does not limit the invention. Instead, the scope of theinvention is defined by the appended claims. The following embodimentsare discussed, for simplicity, with regard to the terminology andstructure of a vibratory source being driven by pseudo-random referencesignals. However, the embodiments to be discussed next are not limitedto plural pseudo-random reference signals but may be applied to a singlepilot signal.

Reference throughout the specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with an embodiment is included inat least one embodiment of the subject matter disclosed. Thus, theappearance of the phrases “in one embodiment” or “in an embodiment” invarious places throughout the specification is not necessarily referringto the same embodiment. Further, the particular features, structures orcharacteristics may be combined in any suitable manner in one or moreembodiments.

In the following, a method for boosting the low-frequency content of thevibratory source is discussed first and a method for boosting thehigh-frequency content of the source is discussed second. In oneexemplary embodiment, the low-frequency boost method may be appliedindependent of the high-frequency boost method. However, in oneapplication, both methods are applied at the same time.

As discussed above, the low-frequency part of the spectrum for avibratory source may be impacted by various characteristics of thesource, e.g., the stroke limitation. A pilot signal may be designed asknown in the art, see for example '804, to be used as an input signalfor a controller of the vibrator. The controller is configured to adjustthe drive signal so that the ground force of the vibrator (e.g., earthcontact force) matches the pilot signal. An example of a pilot signal isa pseudo-random reference signal, as disclosed, for example, in U.S.Pat. No. 7,859,945 (herein '945, the entire content of which isincorporated herein by reference). Thus, a description of thepseudo-random reference signal is omitted herein. For simplicity, in thefollowing, the pilot signal is assumed to include one or morepseudo-random reference signals. However, the novel features describedin this application equally apply to other types of pilot signals, i.e.,pilot signals that contain a mixture or plurality of frequencies at apoint in time.

In general, pseudo-random signals have less energy than a swept sinewave of equal peak amplitude. Thus, it is desirable to apply some typeof wave-shaping to the pilot signal to boost its output. '945 describesa wave-shaping process that is applied to the pilot signal in a forcedomain to increase the root mean square (RMS) content of the groundforce. However, the exemplary embodiment to be discussed next applies afurther wave-shaping to the pilot signal to allow the vibrator to beoperated at a higher drive level setting without mass over travelthereby allowing the vibrator to shake harder thus boosting the outputof the source overall and permitting operation to lower frequency. Thisfurther wave-shaping is applied in the displacement domain in the novelmethod and not in a force domain as in '945.

Considering a vibratory source 10 as shown in FIG. 1, it is noted that aflow drives a reaction mass 12 in one direction, thereby creating areaction force that is transmitted on a baseplate 14 through a rod 13for imparting energy to ground 16. Inside the bore of the reaction mass12 is a double acting piston 15 that is attached through the rod 13 tothe baseplate 14. The baseplate 14 sits on the ground surface 18. Anaccelerometer 12 a may be located on the reaction mass 12 and anaccelerometer 14 a may be located on the baseplate 14. Elements 13, 14,and 15 along with any other elements (e.g., accelerometer 14 a) that arerigidly attached to elements 13, 14, and 15 are referred to as thedriven structure 17. FIG. 1 also shows a hydraulic system 20 thatactuates the reaction mass 12. A servo-valve 22, typically comprised ofa 4-way proportional spool valve that is driven by a small pilot valve,is part of the hydraulic system 20 and regulates an amount of hydraulicfluid that actuates the reaction mass 12 directing flow in and out ofthe interior upper and lower chambers of the reaction mass 12. The pilotvalve converts the electrical drive signal produced by the vibratorcontroller into a hydraulic flow suitable for driving the main stagespool valve. A pump 24 is configured to pump the hydraulic fluid in thehydraulic system 20 and a diesel engine 26 drives the pump 24. Thestroke limitation discussed above with regard to one of the factors thataffects the low-frequency end is related to the relative displacement ofthe reaction mass 12 with respect to the driven structure 17. Thevibratory source 10 also includes a controller 28 configured to controlthe servovalve 22, and thus, the movement of the reaction mass 12. Adisplacement of the reaction mass 12 is D. The controller 28 mayinclude, among other components, a filter 30 for filtering signals asdiscussed later. The controller 28 may be implemented in a computingdevice as discussed later. A linear variable differential transformer(LVDT) 19 is connected between the reaction mass and driven structure toprovide a measure of the relative motion of the reaction mass withrespect to the driven structure and is used as a feedback signal to thevibrator control electronics to help maintain centering of the reactionmass within its stroke range.

In this exemplary embodiment, it is discussed how to boost thelow-frequency end based on the limitations of the vibratory source.Assume that the vibratory source is well-controlled and that the groundforce output of the source matches the pilot signal or that they arerelated by a known scalar, i.e., the ratio of the peak ground force tothe reference peak amplitude. The ground force may be determined by thecontroller 28 as being approximately a weighted sum of the reaction massacceleration and baseplate acceleration, where the weighting factor foreach acceleration is in direct proportion to the mass of the reactionmass 12 and mass of the driven structure 17.

At low-frequencies, e.g., less than 10-Hz, the reaction mass moves muchmore than the baseplate. Because the reaction mass' weight is muchlarger than the weight of the baseplate, at low-frequencies, thereaction mass contributes much more to the weighted sum estimate. In oneapplication, the base-plate's contribution may be ignored.

Thus, to determine the reaction mass displacement D at low frequenciesit is possible to double integrate the pilot signal multiplied by aknown constant of proportionality. The double integral is necessary asthe acceleration of the reaction mass is proportional to the secondorder time derivative of the displacement D. By performing this doubleintegration, the system is “taken” from the force domain to thedisplacement domain.

In this regard, FIG. 2 shows a portion of an exemplary pilot signal 100scaled to force and plotted versus time over a 2 s time interval, FIG. 3shows a first integration of the pilot signal 100 as corresponding to avelocity 102 of the reaction mass plotted versus time, and FIG. 4 showsthe double integration of the pilot signal 100 as corresponding to thedisplacement D of the reaction mass plotted versus time. FIG. 4 alsoindicates stroke limits 104 of the reaction mass 12. FIG. 5 illustratesa prediction of the displacement 106 relative to the measureddisplacement 108. A pure integration operation is marginally stable,i.e., for example, small DC offset errors can lead to large drifts inpredicted motion (the DC gain of an pure integrator is infinite), sotypically a simple single order low pass filter whose corner frequency(a corner frequency of 0.3 Hz for example) is well below the frequenciesof interest is used instead of a pure integration operation.

Being in the displacement domain as illustrated by FIG. 4, a novel levelcompression or wave-shaping step is applied to the displacement asdiscussed next. It is noted that '945 applies a wave-shaping function inthe force domain to the pilot signal but not in the displacement domainto the displacement as discussed here. Also, it is noted that theexemplary embodiment applies the level compression in the displacementdomain in addition to a level compression in the force domain. Thus, theexemplary embodiment applies two level compressions in two differentdomains.

The level compression applied in the displacement domain, as shown inFIG. 4, has the purpose of taking into account the limitations 104 ofthe vibratory source. In other words, if the displacement D of thereaction mass approaches or is beyond the stroke limits 104, the levelcompression modifies the pilot signal to reduce the displacement D belowthe stroke limits 104. The opposite may be applied if portions of thedisplacement D in FIG. 4 are far away from the stroke limits 104, i.e.,the displacement D may be stretched to get closer to the stroke limits104.

An example of level compression is discussed now with regard to FIGS.6A-B and 7. Consider that the displacement D illustrated in FIG. 4 isprovided as input to a level compression function F. The output of thefunction F may be D_(output) and a relation between D_(output) and D isillustrated in FIG. 6A. The original displacement D is illustrated inFIG. 7 as a solid line and the output of the level compression functionis illustrated by D_(output) in FIG. 6A. The level compression functionmay be given by

${{F\left( D_{i} \right)} = {\frac{1}{\left\lbrack {\left( {\frac{D_{i}}{{\delta\sigma}\;{disp}}} \right)^{pwr} + 1} \right\rbrack^{\frac{1}{pwr}}}D_{i}}},$where σdisp is the standard deviation of D, δ is a scalar useful foradjusting the output range function and pwr is a coefficient thatadjusts the curvature at the extremes of F. In general, larger values ofpwr tend to make F look more like a “hard limiter” or clipping functionand smaller values make it perform more like a “soft limiter.” In thisrespect, FIG. 6A shows F for δ=2.5 and pwr=6 and FIG. 6B shows F for δ=3and pwr=2. The compression function used in the '945 is a sine functionand that may be used instead for the level compression of the pilotsignal in the force domain.

After the level compression process in the displacement domain iscompleted, the result is transformed back into the force domain by adouble differentiation with time. Further steps are performed beforeobtaining the desired pilot signal. The desired pilot signal is thenused to drive the vibratory source to generate the seismic waves.

Some of the above steps are now presented in a flowchart as illustratedin FIG. 8. The method starts with step 800 in which a pilot targetamplitude spectrum is defined. Such an amplitude spectrum may be, forexample, a flat spectrum as shown in FIG. 9. Then, in step 802, aninitial pilot signal is spectrally shaped, for example, as in '945, formatching the pilot target amplitude spectrum. The method may be appliedto one or more initial pilot signals. Also, the method is applicable toone or more vibratory sources. The initial pilot signal may be scaled toforce and then a level compression scheme is applied in step 804, in theforce domain to the scaled pilot signal to obtain a compressed pilotsignal. A sine function may be used to implement the level compressionscheme.

The compressed pilot signal is then double integrated in step 806 toarrive to a mass displacement in the displacement domain. The massdisplacement may be the displacement of the reaction mass 12. Having themass displacement, the stroke limits of the vibratory source are usedfor level compression in step 808 to obtain a compressed massdisplacement. As illustrated in FIG. 7, the mass displacement iscorrected for those values that are beyond the stroke limits. Afterthis, in step 810, the compressed mass displacement is doubledifferentiated in time for returning to the force domain to obtain amodified pilot signal. The modified pilot signal may be level compressedin step 812, in the force domain, to obtain the desired pilot signal.This level compression step is next explained.

For a better understanding of the differences between the levelcompression in the displacement domain and the level compression in theforce domain, FIG. 10 shows the original pilot in the force domain andFIG. 11 shows the effect of the displacement domain compression onpilots in the force domain. By comparing FIGS. 10 and 11, it is notedthat picks 120, 122, and 124 in the original pilot signal are reduced inthe modified pilot signal (see 120 a, 122 a and 124 a). An expanded viewof the differences between the original and modified pilot signal areshown in FIG. 12, in which the solid line indicates the original pilotand the dashed line indicates the modified pilot.

FIG. 12 shows that the displacement compression may create some issuesin the force domain. Thus, in order to slow the reaction mass down near,for example, the time t₀=13.34 s in FIG. 12, a large de-accelerationforce is necessary. Therefore, after the displacement is double timedifferentiated and rescaled back in step 810 to arrive in the forcedomain, the method performs another step 812 of level compression in theforce domain so that the peak force at time t₀ is reduced. From here,the method loops back to step 802 until an appropriate pilot signal isobtained. Modifying the pilot signal is an iterative process becausewhen a level compression is applied in one domain it may create issuesin another domain. By repeating the process over and over again, themethod is able to constrain various factors in multiple domains andstill honor the desired target spectrum.

The computer program to design and generate the pilot sweeps discussedabove can be programmed into a personal computer located either in theoffice or in the field. FIG. 16 illustrates a computer system that iswell suited for this application and will be described later. The pilotsweeps can then be stored on a memory stick or other suitable medium.The resultant pilot sweeps may later be transferred or downloaded andstored in sweep tables that reside in the memory of the controller 28 ofthe vibratory source 10 or in a central controller memory (not shown)that coordinates plural vibratory sources of the seismic survey. Whencommanded by either the vibrator operator or the recording truckoperator via a telemetry link, the controller executes the selectedpilot sweep that is stored in its memory.

The method discussed with regard FIG. 8 is appropriate when thelow-frequency end of the vibratory source needs to be boosted. However,another problem for the traditional sources appears for thehigh-frequency end. FIG. 13 shows the maximum peak ground force outputthat can be generated by a small hydraulic vibrator over the range offrequencies that extend up to 300 Hz without exceeding systemconstraints. Note, for example, the drop in output that can be achievedover the range of 120-300 Hz. The low- and high-frequency ends of avibratory source depend upon the characteristic of the source. In otherwords, these ranges change from source to source. However, to illustratethe high-frequency boost, it is assumed that the high-frequency endincludes frequencies between 150 and 300 Hz.

One noted problem for vibratory sources is that the existingpseudo-random signals tend to overdrive the servo-valve of the vibratorysource at high frequencies. In other words, the vibratory hydraulicsystem has certain limitations that limit the ability of the source tocreate high temporal peak levels of high frequencies. Thus, according toan exemplary embodiment, there is a method that lowers a peak temporallevel of the high-frequency components and compensate for the lower peakby increasing the dwell time at those frequencies. This is achieved byredistributing the high-frequency demand so that lower peak levels arespread out over time. In one application, high levels of highfrequencies having short duration is replaced with low levels highfrequencies of a longer duration. For a fixed sweep length, this mayresult in some loss of the low-frequency energy because thelow-frequency duration is traded off for high-frequency duration.

The method to be discussed next may be implemented in the controller 28of the vibratory source 10. In one application, the method may beimplemented in a central controller that controls more than onevibratory source. The method is now discussed with regard to FIG. 14.

In a step 1400, a pilot target amplitude spectrum is defined. Forexample, as shown in FIG. 15A, the pilot target amplitude spectrum maybe flat from 10 to 150 Hz and then rising at a rate of 6 dB/octave from150 to 300 Hz. Of course, other pilot target amplitude spectra may bedefined depending on the vibratory source, the depth of the subsurface,etc. In step 1402, an initial pilot signal is generated. The initialpilot signal may include one or more pseudo-random sweeps or othersignals that are not pseudo-random.

If the initial includes plural pseudo-random sweeps, a step of sweepdecoupling 1404 is applied to reduce cross-correlation between theplural pseudo-random sweeps. However, this step is not necessary if onlyone pilot signal is used. In step 1406 the pilot signal is filtered tomatch the target spectrum, e.g., illustrated in FIG. 15A.

For the next step of the method, the initial pilot signal is dividedinto two or more parts, each part corresponding to a given frequencyband. For example, the pilot signal may be divided into a first partcorresponding to 0 to 150 Hz, a second part corresponding to 150 to 225Hz and a third part corresponding to 225 to 300 Hz. In this example, the225 to 300 Hz is considered to be the high-frequency end of thespectrum. These frequency intervals of the pilot signal may be relatedto frequency intervals 300, 302, and 304 of a profile of the hydraulicsystem response of FIG. 13 now replotted in FIG. 15B on a dB scale.

Thus, the initial pilot signal may be band-pass filtered in step 1408with a filter configured to produce shapes 300 a, 302 a and 304 a asshown in FIGS. 15C-E. If the initial pilot signal includes pluralpseudo-random sweeps, each one is filtered as shown in FIGS. 15C-E. Thespectral shape of each filter 300 a, 302 a and 304 a is in oneapplication complimentary in shape to the constraint (300, 302, and 304)imposed by the hydraulic system (e.g., servo-valve). For example, if itis known that for a vibratory source the servo-valve response drops offby 6 dB when sweeping from 150 Hz up to 225 Hz, the complimentary filterspectrum rises 6 dB over that same frequency interval.

In step 1410, band-passed parts of the initial pilot signal are levelcompressed, for example, with a sine function as described in '945. Instep 1412, the three (more or less parts of the initial pilot signal forother situations) parts of the initial pilot signal are recombined tocreate a recombined pilot signal that covers the desired frequencyrange, e.g., 10 to 300 Hz in FIG. 15A. However, it is noted that therecombination step may introduce unwanted frequencies or may modify inan unwanted way the existing frequencies. Also, the transition regionswhere two parts are joined together may not be smooth.

Thus, in step 1414 a de-emphasis spectral correction is applied to therecombined pilot signal (to each combined sweep if the recombined pilotsignal includes plural sweeps). An example of a de-emphasis filterconfiguration is shown in FIG. 15F. The de-emphasis filter may have ashape that corrects for the changes made in step 1408. In step 1416 theoutput of the step 1414 is filtered to match the target spectrum andfill in gaps in the transition regions between the recombined parts ofthe pilot signal.

Another stage of wave-shaping/level compression may be applied in theforce domain in step 1418 and in step 1420 a low-pass filter may beapplied to remove any spectral content introduced by level compressionthat falls above the highest desired frequency and to obtain the desiredpilot signal. These last steps are optional. In one application, steps1404 to 1420 may be repeated one or more times to better shape (decoupleif plural sweeps are used) the desired pilot signal.

An example of a representative computer system capable of carrying outoperations in accordance with the exemplary embodiments discussed aboveis illustrated in FIG. 16. Hardware, firmware, software or a combinationthereof may be used to perform the various steps and operationsdescribed herein.

The exemplary computer system 1600 suitable for performing theactivities described in the exemplary embodiments may include server1601. Such a server 1601 may include a central processor unit (CPU) 1602coupled to a random access memory (RAM) 1604 and to a read-only memory(ROM) 1606. The ROM 1606 may also be other types of storage media tostore programs, such as programmable ROM (PROM), erasable PROM (EPROM),etc. The processor 1602 may communicate with other internal and externalcomponents through input/output (I/O) circuitry 1608 and bussing 1610,to provide control signals and the like. For example, the processor 1602may communicate with sensors, an actuator system and/or a filter of thevibratory source. The processor 1602 carries out a variety of functionsas is known in the art, as dictated by software and/or firmwareinstructions.

The server 1601 may also include one or more data storage devices,including hard disk drives 1612, CD-ROM drives 1614, and other hardwarecapable of reading and/or storing information such as a DVD, etc. In oneembodiment, software for carrying out the above discussed steps may bestored and distributed on a CD-ROM or DVD 1616, removable media 1618 orother form of media capable of portably storing information. Thesestorage media may be inserted into, and read by, devices such as theCD-ROM drive 1614, the drive 1612, etc. The server 1601 may be coupledto a display 1620, which may be any type of known display orpresentation screen, such as LCD or LED displays, plasma displays,cathode ray tubes (CRT), etc. A user input interface 1622 is provided,including one or more user interface mechanisms such as a mouse,keyboard, microphone, touch pad, touch screen, voice-recognition system,etc.

The server 1601 may be coupled to other computing devices via a network.The server may be part of a larger network configuration as in a globalarea network (GAN) such as the Internet 1628.

As also will be appreciated by one skilled in the art, the exemplaryembodiments may be embodied in a wireless communication device, atelecommunication network, as a method or in a computer program product.Accordingly, the exemplary embodiments may take the form of an entirelyhardware embodiment or an embodiment combining hardware and softwareaspects. Further, the exemplary embodiments may take the form of acomputer program product stored on a computer-readable storage mediumhaving computer-readable instructions embodied in the medium. Anysuitable computer readable medium may be utilized including hard disks,CD-ROMs, digital versatile discs (DVD), optical storage devices, ormagnetic storage devices such a floppy disk or magnetic tape. Othernon-limiting examples of computer readable media include flash-typememories or other known types of memories.

The server 1601 of the computer device 1600 may be configured tospecifically execute the following methods. In one exemplary embodimentillustrated in FIG. 17, there is a method for generating a desired pilotsignal for driving a vibratory source to generate seismic waves. Themethod includes a step 1700 of selecting a pilot target amplitudespectrum for the vibratory source; a step 1702 of determining an initialpilot signal that matches the pilot target amplitude spectrum; a step1704 of scaling the initial pilot signal to a force to be applied by thevibratory source to the ground, wherein the scaled pilot signal is in aforce domain; a step 1706 of level compressing the scaled pilot signalin the force domain to obtain a compressed pilot signal; a step 1708 ofdetermining a mass displacement associated with the vibratory sourcebased on the compressed pilot signal; a step 1710 of level compressingthe mass displacement in a displacement domain to obtain a compressedmass displacement; and a step 1712 of transforming the compressed massdisplacement back to the force domain to obtain a modified pilot signal.The level compressing of the mass displacement takes into account strokelimits of the vibratory source, and the modified pilot signal boosts alow-frequency end of a range of the vibratory source comparative to theinitial pilot signal.

The same server may be used to implement a method for generating adesired pilot signal for driving a vibratory source to generate seismicwaves. The method, illustrated in FIG. 18, includes a step 1800 ofselecting a pilot target amplitude spectrum for the vibratory source; astep 1802 of determining an initial pilot signal that matches the pilottarget amplitude spectrum; a step 1804 of associating the initial pilotsignal with at first and second frequency bands, the second frequencyband including a high-frequency end of a range of the vibratory source;a step 1806 of selecting a first band-pass configuration for the firstfrequency band and a second band-pass configuration for the secondfrequency band; a step 1808 of band-passing a first part of the initialpilot signal associated with the first frequency band with the firstband-pass configuration; a step 1810 of band-passing a second part ofthe initial pilot signal associated with the second frequency band withthe second band-pass configuration; a step 1812 of level compressing thefirst and second parts of the initial pilot signal; a step 1814 ofrecombining the first and second parts of the initial pilot signal toform a recombined pilot signal; and a step 1816 of processing therecombined pilot signal to obtain the desired pilot signal. The desiredpilot signal boosts the high-frequency end of the range of the vibratorysource comparative to the initial pilot signal.

FIGS. 19A-B compare the pilot signal before and after application of thehigh frequency boost with level compression. Both graphs shown in FIGS.19A and B have been band-pass filtered over the frequency band of175-225 Hz. FIG. 19A shows the pilot signal in its first pass throughthe process after step 1406 or 1802. Note the high peaks in the waveformof FIG. 19A as well as intervals where energy is low. FIG. 19 B showsthe band-pass filtered pilot signal waveform after the completion of theiterative process described in FIG. 14 or 18. Note that after the highfrequency boost with level compression process has been completed, thepeaks have been reduced while there are fewer intervals of time with lowenergy.

Referring now to FIG. 20, it shows a graph of the amplitude of the highfrequency boosted pilot after the process illustrated in either FIG. 14or 18 is completed. The amplitude versus frequency plot is displayed ona dB scale. The shape of the graph follows the target spectrumillustrated in FIG. 15A, but in this case a target spectrum was usedwhere the 6 dB/octave boost begins at 100 Hz rather than at 150 Hz.

As before for the low-frequency boost, the pilot sweeps for thehigh-frequency boost can be downloaded onto a suitable memory medium anddownloaded in the field into the memory of the vibrator controller whereit is stored in a part of memory designated as a sweep table. Uponcommand by either the vibrator operator or by the recording truckrecording system operator via telemetry, the vibrator executes theselected pilot sweep stored in its sweep table to provide a referencesignal for the controller to follow so that a useful ground force signalis produced by the hydraulic system thereby imparting the seismic energyinto the ground having the desired spectral content.

The disclosed exemplary embodiments provide a vibratory source and amethod that boosts high- and/or low-frequency content. It should beunderstood that this description is not intended to limit the invention.On the contrary, the exemplary embodiments are intended to coveralternatives, modifications and equivalents, which are included in thespirit and scope of the invention as defined by the appended claims.Further, in the detailed description of the exemplary embodiments,numerous specific details are set forth in order to provide acomprehensive understanding of the claimed invention. However, oneskilled in the art would understand that various embodiments may bepracticed without such specific details.

Although the features and elements of the present exemplary embodimentsare described in the embodiments in particular combinations, eachfeature or element can be used alone without the other features andelements of the embodiments or in various combinations with or withoutother features and elements disclosed herein.

This written description uses examples of the subject matter disclosedto enable any person skilled in the art to practice the same, includingmaking and using any devices or systems and performing any incorporatedmethods. The patentable scope of the subject matter is defined by theclaims, and may include other examples that occur to those skilled inthe art. Such other examples are intended to be within the scope of theclaims.

What is claimed is:
 1. A method for generating with a computing device adesired pilot signal for driving a vibratory source to generate seismicwaves, the method comprising: selecting a pilot target amplitudespectrum for the vibratory source; determining an initial pilot signalthat matches the pilot target amplitude spectrum; associating theinitial pilot signal with first and second frequency bands, the secondfrequency band including a high-frequency end of a range of thevibratory source; selecting a first band-pass configuration for thefirst frequency band and a second band-pass configuration for the secondfrequency band; band-passing a first part of the initial pilot signalassociated with the first frequency band with the first band-passconfiguration; band-passing a second part of the initial pilot signalassociated with the second frequency band with the second band-passconfiguration; level compressing the first and second parts of theinitial pilot signal; recombining the first and second parts of theinitial pilot signal to form a recombined pilot signal; and processingthe recombined pilot signal to obtain the desired pilot signal, whereinthe desired pilot signal boosts the high-frequency end of the range ofthe vibratory source comparative to the initial pilot signal.
 2. Themethod of claim 1, wherein the second band-pass configuration iscomplimentary to a response of a hydraulic system which is part of thevibratory source.
 3. The method of claim 1, further comprising: applyinga de-emphasis spectral correction on the modified pilot signal.
 4. Themethod of claim 3, further comprising: filtering out unwantedfrequencies of the recombined pilot signal to obtain a filtered pilotsignal.
 5. The method of claim 4, further comprising: level compressingthe filtered pilot signal to obtain the desired pilot signal.
 6. Themethod of claim 5, further comprising: applying a low-pass filter to thedesired pilot signal.
 7. A controller configured to generate a desiredpilot signal for driving a vibratory source to generate seismic waves,the controller comprising: an interface configured to receive a pilottarget amplitude spectrum for the vibratory source; and a processorconnected to the interface and configured to, determine an initial pilotsignal that matches the pilot target amplitude spectrum, associate theinitial pilot signal with at first and second frequency bands, thesecond frequency band including a high-frequency end of a range of thevibratory source, select a first band-pass configuration for the firstfrequency band and a second band-pass configuration for the secondfrequency band, band-pass a first part of the initial pilot signalassociated with the first frequency band with the first band-passconfiguration, band-pass a second part of the initial pilot signalassociated with the second frequency band with the second band-passconfiguration, level compress the first and second parts of the initialpilot signal, recombine the first and second parts of the initial pilotsignal to form a recombined pilot signal, and process the recombinedpilot signal to obtain the desired pilot signal, wherein the desiredpilot signal boosts the high-frequency end of the range of the vibratorysource comparative to the initial pilot signal.
 8. The controller ofclaim 7, wherein the second band-pass configuration is complimentary toa response of a hydraulic system which is part of the vibratory source.9. The controller of claim 7, wherein the processor is furtherconfigured to: apply a de-emphasis spectral correction on the modifiedpilot signal.
 10. The controller of claim 9, wherein the processor isfurther configured to: filter out unwanted frequencies of the recombinedpilot signal to obtain a filtered pilot signal.
 11. The controller ofclaim 10, wherein the processor is further configured to: level compressthe filtered pilot signal to obtain the desired pilot signal.
 12. Thecontroller of claim 11, wherein the processor is further configured to:apply a low-pass filter to the desired pilot signal.
 13. Anon-transitory computer readable medium including computer executableinstructions, wherein the instructions, when executed by a processor,implement instructions for generating a desired pilot signal for drivinga vibratory source to generate seismic waves, the instructionscomprising: selecting a pilot target amplitude spectrum for thevibratory source; determining an initial pilot signal that matches thepilot target amplitude spectrum; associating the initial pilot signalwith at first and second frequency bands, the second frequency bandincluding a high-frequency end of a range of the vibratory source;selecting a first band-pass configuration for the first frequency bandand a second band-pass configuration for the second frequency band;band-passing a first part of the initial pilot signal associated withthe first frequency band with the first band-pass configuration;band-passing a second part of the initial pilot signal associated withthe second frequency band with the second band-pass configuration; levelcompressing the first and second parts of the initial pilot signal;recombining the first and second parts of the initial pilot signal toform a recombined pilot signal; and processing the recombined pilotsignal to obtain the desired pilot signal, wherein the desired pilotsignal boosts the high-frequency end of the range of the vibratorysource comparative to the initial pilot signal.
 14. The medium of claim13, wherein the second band-pass configuration is complimentary to aresponse of a hydraulic system which is part of the vibratory source.15. The medium of claim 13, further comprising: applying a de-emphasisspectral correction on the modified pilot signal.
 16. The medium ofclaim 15, further comprising: filtering out unwanted frequencies of therecombined pilot signal to obtain a filtered pilot signal.
 17. Themedium of claim 16, further comprising: level compressing the filteredpilot signal to obtain the desired pilot signal.
 18. The medium of claim17, further comprising: applying a low-pass filter to the desired pilotsignal.